The invention relates to transcoding of a data stream comprising a time multiplex of coded data and control data. The data stream may represent, for example, a sequence of images which have been coded in accordance with an image-coding standard such as, for example, defined by the Moving Pictures Experts Group (MPEG).
EP-A 0 690 392 describes a method of transcoding coded digital signals corresponding to a sequence of images. The method comprises a decoding step of the input digital signals which are associated with each current image. The decoding step is followed by an encoding step.
It is an object of the invention to provide transcoding of a data stream which yields a satisfactory quality. Claims 1, 4 and 6 define a method of transcoding, a transcoder, and a transcoded data stream, respectively, in accordance with the invention.
The invention takes the following aspects into consideration. A data stream may not only comprise coded data, but also control data. In transcoding, the data stream will effectively be split up into a coded data sub-stream and a control data sub-stream. The coded data sub-stream is transcoded, resulting in a transcoded data sub-stream. It may be necessary to adapt the control data for the transcoded data. In that case, the control data sub-stream is effectively transformed into an adapted control data sub-stream. The transcoded data sub-stream and the adapted control data sub-stream may be combined so as to obtain a transcoded data stream which is a different representation of data originally contained in the coded data stream.
Some coding standards, such as, for example, MPEG image-coding standards do not fix the size of the coded data but implicitly fix the size of the control data. In such a coding standard, coded data may be decoded and re-coded so as to obtain transcoded data which may differ in size from the coded data. However, it is not possible to do the same for the control data without departing from the coding standard. If the data stream has to be transcoded while maintaining conformity with its coding standard, one may adapt the content of the control data but not its size. As a result, the transcoded data sub-stream and the adapted control data sub-stream will have different bit rates.
In accordance with the invention, the transcoded data and the adapted control data are written into a transcoder output buffer and read from the transcoder output buffer so as to obtain a transcoded data stream. Accordingly, any difference in bit rate between the transcoded data sub-stream and the control data sub-stream can be averaged out in such a way that the transcoded data stream has a bit rate which is substantially constant or, at least, which does not substantially depend on whether coded data or control data is conveyed. Thus, the invention allows the bit rate of the transcoded data stream to be continuously equal to a maximum bit rate which a certain transmission channel can handle or, at least, relatively close thereto. Consequently, since the invention allows an efficient use of the transmission channel via which the transcoded data stream is transmitted, it allows the transcoded data to have a relatively large size. Accordingly, the invention ensures that a maximal amount of data is preserved in the transcoding. Thus, the invention allows transcoding with a satisfactory quality.
The invention and additional features, which may be optionally used to implement the invention to advantage, are apparent from and will be elucidated with reference to the drawings described hereinafter.